Local Area Networks (LANs) provide a method for connecting computers or other devices together to exchange data or to harness groups of computers together to apply their combined power to a single problem. Generally speaking, a LAN includes: 1) a high speed transmission medium, typically metallic or fiber optic, for connecting each of the devices to the LAN; 2) the ability to transmit a message on the transmission medium directed to a single device; and 3) a means known as "broadcast" in which all devices connected to the LAN medium can receive a message transmitted on the medium. A standard for the implementation of LAN devices and systems has been established by the Institute of Electrical and Electronic Engineers as IEEE Standard 802.
The physical length of the transmission medium and the total number of devices connected thereto are typically limited on a LAN due to the physics of high speed transmission systems. Bridges and routers are devices used to connect multiple LANs to provide communications between individual LANs and to construct large networks that transcend the technical size limits of a single individual LAN. When the individual LANs to be interconnected are at geographically remote locations, bridges and routers are used in pairs, one at each site, to provide a path for data to flow from one LAN to another, with a lower speed communication link between the bridge or router pair. Typically the data rates of the long distance communications link is a fraction of the data rate of the LAN medium. The use of bridges and routers has been limited, however, due to the cost of these devices and the costs of the long distance communications link.
An all-digital telephone network, known as the Integrated Services Digital Network ("ISDN"), has become a potential substitute for the private long distance lines currently used by bridges and routers. ISDN provides relatively high speed digital transmission service on an "as needed" basis, and is different from LAN transmission media in that it is a switched transmission media which provides a point-to-point transmission service on an intermittent basis.
Modern communications technology can be analyzed with respect to the Open Systems Interconnect (OSI) Reference Model. The OSI model decomposes a communication system into seven major components or layers which are defined by international standards. The OSI model is concerned with the interconnection between systems, i.e., the way they exchange information, and not with the internal functions that are performed by a given system. The OSI model depicted in FIG. 1 provides a generalized view of a layered architecture, using an approach where sets of functions have been allocated to different layers.
The first layer is known as the physical layer and is responsible for the transmission of bit streams across a particular physical transmission medium. This layer involves a connection between two machines that allows electrical signals to be exchanged between them.
The second layer is the data link layer, and is responsible for providing reliable data transmission from one node to another and for shielding higher layers from any concerns about the physical transmission medium. It is concerned with the error-free transmission of frames of data.
The third layer, the network layer, is concerned with routing data from one network node to another and is responsible for establishing, maintaining, and terminating the network connection between two users and for transferring data along that connection. There can be only one network connection between two given users, although there can be many possible routes from which to choose when the particular connection is established.
The fourth layer is the transport layer, and is responsible for providing data transfer between two users at an agreed on level of quality. When a connection is established between two users, the transport layer is responsible for selecting a particular class of service to be used, for monitoring transmissions to ensure the appropriate service quality is maintained, and for notifying the users if it is not.
The fifth layer i the session layer, and it focuses on providing services used to organize and synchronize the dialog that takes place between users and to manage the data exchange. A primary concern of the session layer is controlling when users can send and receive, based on whether they can send and receive concurrently or alternately.
The sixth layer is the presentation layer, and is responsible for the presentation of information in a way that is meaningful to network users. This may include character code translation, data conversion or data compression and expansion.
The seventh layer is the application layer, and it provides a means for application processes to access the system interconnection facilities in order to exchange information. This includes services used to establish and terminate the connections between users and to monitor and manage the systems being interconnected and the various resources they employ.
Different components (or implementations) that conform to a common standard are considered equivalent and interchangeable. A system constructed from components that conform to their respective standard is expected to interoperate (i.e., to be able to communicate) with any other system constructed out of a different set of components that conform to the standards. Communications between systems are organized into information that is exchanged between entities at each layer.
A layer in the OSI model provides specific services to an upper layer through service access points ("SAPs"). Take, for example, the situation where Systems A and B are joined by a transmission medium at layer 1. Information from layer x of system A is constrained to communicate with layer x of system B. The information of layer x of system A is transported, however, by requesting service from layer x-1 of system A for delivery to layer x of system B. The mechanism for communication between two systems at a single layer is referred to as a protocol (i.e., "a layer x protocol"), and a protocol stack is a set of protocols for layers 1 to x. The OSI protocols provide flexibility in usage by incorporating optional features and user determined parameters. Profiles are standards that specify the selection of options and parameters to ensure compatibility between two compliant systems. Profiles are needed since two compliant systems using different profiles may still not be able to exchange data.
In FIG. 1, layer 1 represents the network or transmission medium, and includes token rings, token buses, and interfaces such as RS-232, RS-530 and V.35. Layer 2, the data link layer, has as its primary responsibility the transfer of frames of information between physically linked devices. When only two devices are connected by the network layer medium, the data link layer assumes that the network layer will provide the mechanism of addressing messages to the proper device.
IEEE Standard 802.2 provides a model which divides the data link layer 2 into two sublayers: an upper sublayer for Logical Link Control (LLC) and a lower sublayer for Media Access Control
The IEEE 802.2 model differs from earlier data link layers of the OSI Reference Model by providing a method for addressing messages to specific destinations. This is required since more than two devices are connected by the medium at layer 1.
This mechanism is necessary in the context of a single isolated LAN (or LAN segment) without connections to other LANs (or LAN segments) because many devices are connected to a common transmission medium and a means for directing a message to a single destination is important.
The MAC sublayer regulates station access to the transmission medium that is shared by multiple stations on the LAN. For a given LAN, the MAC sublayer governs a common transmission medium that has one pathway or route between communicating network stations. In the context of the IEEE 802.2 model, the network station address is referred to as the MAC address and is sufficient for ensuring delivery of a MAC frame to a destination address on the LAN. The MAC sublayer offers services consistent with those in the OSI data link layer.
The LLC sublayer mediates multiple logical connections for upper layer service users. As a service provider, the LLC sublayer offers several Service Access Points (SAP) as logical ports for multiple upper layer entities located at a given network station address. As a service user, the LLC sublayer issues requests through the SAP provided by the MAC sublayer. The LLC sublayer Service Access Points are typically shown situated between layer 3 (network) and layer 2 (data link) of the OSI Reference Model.
A significant number of layer 3 protocols bypass the LLC Service Access Point and interface directly to the MAC Service Access Point.